Why is a DUNS number required for me to register? On the registration page, it tells me that it is a required field and my company already has registered with our DUNS number.

The customer that invited you has the Do not require suppliers to enter their D-U-N-S number disabled in their Default Transaction Rules. This is also known as the D-U-N-S Number Rule. To resolve this, you can contact your customer to discuss a rule change or log in with an existing account your company has.

SAP Business Network for Procurement & Supply Chain > Administration (Buyer & Supplier Account Administration)